Hitherto, in a high-frequency dielectric heating apparatus using a magnetron such as a microwave oven, electric power supplied to the magnetron has been adjusted based on the pulse width of output signal from an inverter controller. To increase the output power, the pulse width of the output signal form the inverter controller has been widened, thereby increasing the electric power supplied to the magnetron. This configuration makes it possible to continuously change heat output of the magnetron.
FIG. 6 is a block diagram including a magnetron used in a high-frequency dielectric heating apparatus in a prior art. In FIG. 6, alternating current from a commercial power supply 11 is rectified to direct current by a rectifier 13 and the direct current is smoothed through a choke coil 14 on the output side of the rectifier 13 and a smoothing capacitor 15 and is given to the input side of an inverter 16. The direct current is converted into any desired high frequency (20 to 40 kHz) by turning on/off a semiconductor switching element, IGBT (Insulated Gate Bipolar Transistor) in the inverter 16. The inverter 16 includes IGBT 16a for switching the direct current at high speed and a capacitor 16b and is driven by an inverter controller 161 for controlling the inverter 16 to switch on/off electric current flowing through the primary side of a boosting transformer 18 at high speed.
In the boosting transformer 18, high-frequency voltage output by the inverter 16 is applied to a primary winding 181 and a high voltage responsive to the turns ratio is obtained at a secondary winding 182. A winding 183 with a smaller number of turns is provided on the secondary side of the boosting transformer 18 and is used to heat a filament (heater, cathode) 121 of a magnetron 12. The secondary winding 182 of the boosting transformer 18 includes a voltage-doubling rectifier 20 for rectifying output of the secondary winding. The voltage-doubling rectifier 20 is made up of high voltage capacitors 201 and 202 and two high voltage diodes 203 and 204.
A thermistor 9 for detecting the temperature of the IGBT 16a is attached directly to a leg part of the IGBT 16a or the proximity of the leg part. The leg part is an emitter leg and a chip (chip thermistor) forming the thermistor is soldered on the solder face of the back of a printed board 6 (FIG. 7) rather than the radiation fin side. Temperature information provided by the thermistor is input to the inverter controller 161 and is used to control the inverter 16.
FIG. 7 shows the printed board 6 on which a radiation fin 7, the IGBT 8 (16a), and the thermistor 9 are placed. The heating part of the IGBT 8 for generating high heat is fixed to the radiation fin 7 and three legs are inserted into through holes of the printed board and are soldered on the opposite side (back, solder side). The chip thermistor is used as the thermistor 9 and is soldered directly to the legs of the IGBT 16a on the solder face of the back of the printed board 6 rather than the radiation fin side.
In the configuration, a method of preventing thermal destruction of the IGBT for switching the inverter power supply, so-called power down control of stopping or powering down before thermal destruction of the IGBT to prevent a temperature rise is executed. An outline of the power down control is as follows:
(1) When the IGBT temperature reaches a detection temperature, first the power is decreased to a first predetermined value (for example, about a half) without suddenly turning off the power. Then, when the IGBT temperature lowers and falls below the detection temperature, again the power is restored to predetermined power and when the IGBT temperature rises and again reaches the detection temperature, again powering down is performed. This operation sequence is repeated for keeping the detection temperature.
(2) A given control width signal is always given from a microcomputer, and in the inverter, the thermistor detects the temperature of the IGBT and sends a detection value to the inverter controller for controlling the inverter so as to lower the temperature of the IGBT.
(3) The thermistor is previously inserted into one of partial pressure circuits and gradual decrease control is performed based on the partial pressure ratio when the thermistor detects overheat temperature.
(4) When the gradual decrease control reaches at a certain point in time, the target value is largely decreased and this control is repeated. One cycle of the gradual decrease control is about one to two seconds at shortest. Such control is made possible by lessening a heat time constant as the chip thermistor is provided on the terminal back of the IGBT as described above.
Therefore, even when a foreign substance is caught in a fan for some reason and the fan abruptly stops, immediately cooking is stopped. In the configuration, however, attention is focused on the fact that thermal destruction of the IGBT does not easily occur if the fan fails, and cooking is allowed to continue. When the temperature of the IGBT rises and reaches a temperature before the temperature at which thermal destruction of the IGBT occurs, then the power is decreased to about a half and heating is allowed to continue. In such control, if the cooking is ordinarily performed, the user feels that warming is a little slow, and can be allowed to continue cooking without feelings of anxiety of a failure; mental anxiety can be circumvented.
Similar comments are also applied when the fan is locked; the heating operation can be allowed to continue at the minimum output to such an extent that the IGBT is not thermally destroyed without shutting down the power.
FIGS. 8A and 8B are drawings to describe the power down control system described above; FIG. 8A is a circuit diagram and FIG. 8B is a timing chart showing the operation of a comparator.
In FIG. 8A, the potential at a point P3 provided by dividing the collector voltage of IGBT using partial pressure resistors R3 and R4 is input to a (A) terminal of one of two input terminals of a comparator CO1 and during starting, a changeover switch S1 is at the position of an a terminal and 3 V is applied to the other (C) terminal. When a magnetron heats and enters a stationary state (stationary operation) from the starting, the changeover switch S1 switches to the position of a b terminal and the potential at a Pc point provided by dividing Vcc voltage using a partial pressure resistor R1 and a thermistor T1 (reference numeral 9 in FIGS. 6 and 7) is input.
Since the thermistor T1 has a characteristic that the resistance value decreases with a temperature rise, when the detection temperature of the thermistor is a predetermined value, the collector voltage gradually decreases from the point like (C) in FIG. 8B. During the starting, the inverter controller 161 controls the ON/OFF duty of the IGBT so that the P3 potential roughly matches 3 V based on ON/OFF information of the comparator CO1 and thus the collector voltage of the IGBT becomes lower than that at the stationary time. When the stationary state is entered, sufficiently high Pc potential as compared with 3 V at the starting time is input to the (C) terminal of the comparator CO1. Therefore, the inverter controller 161 increases the ON duty of ON/OFF control of the IGBT so that the P3 potential (A) roughly matches the Pc potential (C), and the collector voltage of the IGBT also increases. However, although not shown in the figure, a limitation is imposed on the rise in the ON duty mentioned above by a power control function of controlling based on another input signal, included in the inverter controller 161 and thus the Pc potential (C) always becomes higher than the P3 potential (A) and output of the comparator CO1 is maintained on at all times, as shown in FIG. 8B. However, since the resistance of the thermistor T1 decreases with heating of the IGBT because of cooling shortage, etc., when the Pc potential (C) becomes the same as the P3 potential (A), again ON/OFF is started. The inverter controller 161 decreases the ON duty of ON/OFF control of the IGBT so that the P3 potential (A) decreases following the decrease in the Pc potential (C) and thus output of the inverter decreases.
Thus, in the power down control, the inverter section starts the magnetron and after the stationary state is entered, output voltage of the inverter section is made to depend on the resistance value of the thermistor and thus if the fan does not rotate for some reason, the inverter section is allowed to continue the operation without shutting down the power. As the temperature of the IGBT rises, the resistance value of the thermistor decreases and output of the inverter decreases, so that the user can continue cooking with such a feeling that the heating temperature is a little low.    Patent document 1: JP-A-2004-327123